1. Field of the Invention
The present invention relates to a honeycomb structure.
2. Discussion of the Background
In recent years, particulates (hereinafter, also referred to as “PM”) such as soot and other toxic components contained in exhaust gases discharged from internal combustion engines of vehicles such as buses and trucks, construction machines and the like has raised serious problems as contaminants harmful to the environment and the human body. For this reason, various honeycomb structures made of porous ceramics have been proposed as honeycomb filters for collecting PM in exhaust gases to purify the exhaust gases.
Conventionally-known honeycomb structures as above described include a honeycomb structure having a ceramic block including a combination of a plurality of honeycomb fired bodies each having a large number of cells (JP-A 2004-154718). FIGS. 1A and 1B set forth herein schematically illustrate examples of the honeycomb fired bodies positioned at the outermost periphery of the honeycomb structure, among honeycomb fired bodies used to manufacture a conventional honeycomb structure disclosed in JP-A 2004-154718. Honeycomb fired bodies 1110 and 1120 illustrated in FIGS. 1A and 1B respectively have cells 1111 and 1121 positioned closest to the curved surface forming the peripheral face of the honeycomb block. The shapes of the cells 1111 and 1121 in a cross section perpendicular to the longitudinal direction of the cells (hereinafter, simply referred to as cross-sectional shape) respectively have an approximate triangular shape and an approximate trapezoidal shape, which are different from the cross-sectional shapes of cells positioned under the cells 1111 and 1121. Each of the cells 1111 and 1121 has a side formed along the curved surface.
A honeycomb structure as disclosed in WO 2008/126335 A1 has been proposed as a honeycomb structure in which cells of a honeycomb fired body are more likely to be surely sealed without defective filling. WO 2008/126335 A1 discloses a honeycomb structure which facilitates filling of cells with a plug material paste by the following configuration. Namely, in a honeycomb fired body included in the honeycomb structure, the cross-sectional shape of a cell contacting with a peripheral wall forming the periphery of a ceramic block (hereinafter, also referred to as a cell positioned at the outermost periphery), among peripheral walls of the honeycomb fired body, and the cross sectional shape of a cell not positioned at the outermost periphery are the same.
FIGS. 2A and 2B set forth herein show examples of honeycomb fired bodies included in the conventional honeycomb structure disclosed in WO 2008/126335 A1, in which the cross-sectional shape of a cell positioned at the outermost periphery and the cross sectional shape of a cell not positioned at the outermost periphery are the same. Cells 1151 in a honeycomb fired body 1150 and cells 1161 in a honeycomb fired body 1160 all have the square cross-sectional shape. Further, the cells 1151 and 1161 are formed so as to be positioned at equal intervals in the honeycomb fired bodies 1150 and 1160, respectively. In order to make the cell positioned at the outermost periphery and the cell not positioned at the outermost periphery have the same cross-sectional shape, a peripheral wall 1154 in the honeycomb fired body 1150 and a peripheral wall 1164 of the honeycomb fired body 1160 respectively have level differences at positions corresponding to the cells 1151 and 1161 positioned at the outermost periphery.
The contents of JP-A 2004-154718 and WO 2008/126335 A1 are incorporated herein by reference in their entirety.